| Summary: | With the development of controlled and predictable nanoparticle fabrication, assembling multiple nano-objects into larger functional nanostructure has attracted increasing attention. As the most basic structure, assembly of one-dimensional (1D) structures is a good model for investigating the assembly mechanism of a nanostructure's formation from individual particles. In this dissertation, the dynamics and the growth mechanism of anisotropic 1D nanostructures is investigated.
In our first study, we demonstrate a simple method for assembling superparamagnetic nanoparticles (SPIONs) into structure-controlled 1D chains in a rotating magnetic field. The length of the SPION chains can be well described by an exponential distribution, as is also seen in SPION chains in a static field. In addition, the maximum chain length is limited by the field's rotational speed, as is seen in micro-sized beads forming chains in a rotating field. However, due to a combination of thermal fluctuations and hydrodynamic forces, the chain length in our case is shorter than either limit. In addition to chain length, the disorder of chains was also studied. Because of the friction between particles, kinetic potential traps prevent relaxation to the global free energy minimum. The traps are too deep to be overcome through thermal fluctuations, and assemblies captured by the kinetic traps therefore form disordered chains. We demonstrate that this disorder gradually heals over a timescale of tens of minutes and that the healing process can be promoted by increasing particle concentration or solution ionic strength, suggesting that the chain growth process provides the energy required to overcome the kinetic trapping.
Next, we introduce a novel optical technique we term Quantitative Optical Anisotropy Imaging (QOAI). A fast and precise single-particle characterizing technique for anisotropic nanostructures, QOAI allows real-time tracking of particle orientation as well as the spectroscopic characterization of polarizabilities of nanoparticles on a microsecond timescale. The abilities of QOAI are demonstrated by the detection and the characterization of single gold nanorods. We also show that single particle diffusions and the process of particle binding to a wall can be tracked through QOAI. The rotational diffusivities of gold nanorods near the wall were determined by autocorrelation analysis, which shows that the diffusivity in the polar direction is slightly smaller than in the azimuthal direction. This result demonstrates that a detailed correlation analysis with QOAI may provide the opportunity to analyze both the translational and rotational motion of particles simultaneously, enabling true 3-dimensional orientation tracking.
Finally, optical methods including QOAI are applied to the investigation of magnetic assembly, demonstrating that optical anisotropy is generated during particle binding, which can be used as a probe of the magnetic assembly process. QOAI is employed to track the dynamics of magnetic clusters in real time, attempting to capture insights on the self-assembly of the magnetic nanoparticles. By turning the external magnetic field on and off, the processes of combining superparamagnetic colloidal nanoparticle clusters into chain assemblies are monitored along with the chain growth. This fast and orientation-sensitive single-particle measurement opens the door to detailed studies of self-assembly away from equilibrium. === Doctor of Philosophy === Nanotechnology is the study and application of phenomena at the nanoscale, which is between 1 and 100 nm. Due to quantum effects, nanomaterials exhibit many interesting properties that cannot be found in bulk materials and are highly influenced by the shape of the nanostructures. One of the most promising strategies for forming complex nanostructures is to use smaller nanoparticles as building blocks. Therefore, significant efforts have been spent on the studies of the fabrication and modeling of the assembly of nanostructures.
As a good starting point for analyzing the mechanism of self-assembly, we focus on the most basic structure, one-dimensional (1D) nanowires and chains. First, we demonstrate a simple method to fabricate one-dimensional magnetic chains from spherical magnetic nanoparticles in a rotating magnetic field. The growth mechanism of the nanochains is investigated, indicating the theory developed for chains formed with larger beads is not applicable at the nanoscale, and additional factors, such as the effect of temperature, need to be considered. Second, we introduce a fast, sensitive optical technique for characterizing anisotropic nanostructures. Because of their unique optical properties, gold nanorods are used to demonstrate the capabilities of the optical system. Not only static properties (orientation, aspect ratio), but also dynamics properties (rotational motion), of single gold nanorods are characterized quantitatively. Finally, this optical technique is extended to preliminary work on characterizing magnetic chain assembly. The processes of magnetic cluster binding and dissociation in a magnetic field are monitored and analyzed.
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